Metabolism of L-[sulfane-34S]thiocystine by Escherichia coli

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Abstract
The metabolism of L-thiocystine [bis(2-amino-2-carboxyethyl) trisulfide] by E. coli was studied by using L-[sulfane-35S]thiocystine. This compound was found to serve as a source of S for E. coli grown on a defined medium free of other S sources and to incorporate its labeled S into cysteine as well as the other S-containing cellular components. For determination of the extent of the synthesis of new cysteine in these cells, cells were grown with [3,3-2H2]serine and L-[sulfane-34S]thiocystine, and the extent of incorporation of both deuterium and 34S into the cellular cysteine was measured by gas chromatography-mass spectrometry. The results show that .apprx. 50% of the cysteine which is incorporated into cellular macromolecules is derived from the thiocystine without cleavage of the C-S bond, the remaining portion being newly biosynthesized from serine and 34S-enriched H2S. The 1st step in the metabolism of thiocystine by E. coli apparently involves the .beta. elimination of pyruvate. This type of reaction is characteristic of the cleavage reactions catalyzed by .beta.-cystathionase.